1. Field of the Invention
This invention relates to a method for treating wastewater and more specifically, to a method for treating wastewater containing phenolic compounds and dispersed polycarbonate particles from an interfacial polycarbonate manufacturing process.
2. Description of Related Art
In one method for the preparation of polycarbonate, bisphenol A is reacted with phosgene using a two-phase reaction mixture. Using this interfacial polycondensation technique, one phase is an aqueous, alkaline phase and the other phase is an organic liquid phase. In this method, bisphenol A is typically dissolved as a salt (for example, sodium bisphenolate) in the aqueous phase and phosgene is dissolved in the organic phase. The organic phase generally comprises a chlorinated hydrocarbon such as dichloromethane as the organic liquid, reaction medium. The phosgene and bisphenol react at the interface of the two phases to produce carbonate oligomers with reactive chloroformate end groups which enter the organic phase while the salts formed by the reaction (for example, NaCl and Na2CO3) enter the aqueous phase of the reaction mixture.
After formation of carbonate oligomers with reactive end-groups, a catalyst, typically a tertiary aliphatic amine such as triethylamine, is added to accelerate polycondensation of oligomers to higher molecular weight products in a so-called coupling step. Monofunctional phenols may also be added to the reacting mixture during or after the phosgenation or in the coupling step to control molecular weight. Multifunctional phenols may also be added to the polymerization process to act as a branching agent.
After the polycondensation reaction, the aqueous phase is separated from the organic phase and the organic and aqueous phases subsequently treated. Typically, the organic phase is treated by washing with a dilute acid such as hydrochloric or phosphoric acid, to extract the amine coupling catalyst into the acid. Electrolytes are then removed from the organic phase by washing with demineralized water. The wash water used in this washing step requires subsequent purification prior to reuse or disposal.
The aqueous phase is conventionally purified by stream stripping to remove residual solvent and the coupling catalyst and then treated to remove residual amounts of polymer, water soluble polycarbonate oligomers, polycarbonate particles and/or bisphenol such as in a biological wastewater facility or by absorption with activated carbon or ion-exchange resins.
Unfortunately, the phenolic materials are relatively resistant to biological reactions. Thus, relatively long times are required in the biological waste treatment units. In addition, purification by adsorption is limited by the adsorption capacity of the activated carbon for phenolic bodies. The regeneration of the carbon is difficult. In addition, water soluble polycarbonate oligomers often cannot be removed by activated carbon. The use of ion exchange resins is limited by the fact that the wastewater contains other salts such as sodium chloride and sodium carbonate in relatively high concentrations. In addition, small polycarbonate particles are not effectively removed by the adsorption or ion-exchange methods.
An alternative method for removing phenolic bodies from the water is an extraction method described in DE 195 10063. In this patent, the described extraction uses an amine dissolved in a hydrocarbon or a mixture of hydrocarbons. The process includes a regeneration step of the hydrocarbon phase by alkaline extraction. However, additional regeneration steps are required to separate the extracted phenolic bodies from the amine solution. In addition, the extraction method using a hydrocarbon is not suitable to remove polycarbonate particles, dispersed in the water phase, as polycarbonate is nearly insoluble in hydrocarbons.
It remains desirable to purify the water used in the preparation of polycarbonate using more effective means.